Not true for this year for sure, international at least. I booked international a few weeks out--because the price was the lowest J class I have ever seen (it is a route I monitor annually, with multiple options). In the past month alone I have watched it fall by $1K. The economy in the US and trade wars, etc. are part of it, not to mention myriad factors that play in all the time. That said I also bought a regular ticket for work that I buy often, familiar route. I realized suddenly it was 14 days from the flight. The price was steady, usual, and what it had been almost since I looked the first time. A medium distance AA huh-to-hub.

As with everything, YMMV!